Electrochemical devices such as a lithium secondary battery are characterized by a high energy density, and thus have been widely used as power sources for portable equipment such as a portable telephone and a notebook personal computer. For example, the capacity of the lithium secondary battery is likely to increase further as the performance of the portable equipment becomes higher. For this reason, it is important to ensure the safety of the lithium secondary battery
In the current lithium secondary battery, for example, a polyolefin-based microporous film having a thickness of about 20 to 30 μm is used as a separator that is interposed between a positive electrode and a negative electrode. Polyethylene with a low melting point is used in some cases as the material of a separator to ensure a so-called shutdown effect. The shutdown effect improves the safety of the battery in the event of a short circuit by allowing the resin constituting the separator to melt at temperatures equal to or lower than the thermal runaway temperature of the battery to close the pores to increase the internal resistance of the battery
A uniaxially—or biaxially—oriented film is used for be separator to improve the porosity and strength. Since such a separator is provided as an independent film, it needs to have a certain level of strength in view of workability, and the drawing ensures the strength of the separator. In such a uniaxially—or biaxially—oriented film, however, the degree of crystallinity is increased, and the shutdown temperature is also raised close to the thermal runaway temperature of the battery. Thus, it is hard to say that the margin for safety of the battery is sufficient.
Moreover, the film has been distorted by drawing and may shrink due to residual stress when it is subjected to high temperatures. The shrinkage temperature is very dose to the melting point, namely the shutdown temperature. Therefore, when the polyolefin-based microporous film is used as a separator, a rise in temperature of the battery has to be prevented by reducing the current as soon as the temperature of the battery reaches the shutdown temperature due to charging anomaly or the like If the pores are not sufficiently closed and the current cannot be immediately reduced, the temperature of the battery is easily raised to the shrinkage temperature of the separator, so that an internal short circuit can occur.
As a technique to prevent such a short circuit caused by thermal shrinkage of a separator to improve the reliability of a battery, for example, it is proposed to use a porous separator having a first separator layer containing a resin for ensuring the shutdown function as the main component and a second separator layer containing a filler having a heat-resistant temperature of 150° C. or higher as the main component in forming an electrochemical device (Patent document 1).
By the technique of Patent document 1, it is possible to provide an electrochemical device, such as a lithium secondary battery, that has an excellent level of safety and does not exhibit thermal runaway even when the element is overheated anomalously.
Further, it has been contemplated in a variety of ways to make improvements to electrochemical devices such as a lithium secondary battery in characteristics other than the safety: For example, Patent documents 2 and 3 disclose that the use of a negative electrode active material whose surface is coated with a low-crystalline carbon material allows an increase in the capacity. It also allows a reduction in the irreversible capacity at the initial charge-discharge cycle, so that the charge-discharge cycle capacity retention rate can be increased to significantly improve quick charge characteristics